The present invention concerns a trigger probe, ordinarily used on a measuring machine and serving to detect the contact between the probe""s stylus and a piece to be measured. The present invention also concerns a method for assembling such a probe.
The content of Application No. EP01811164.1, filed on Nov. 30, 2001 with the European Patent Office, is hereby incorporated by reference.
Trigger probes are electromechanical measuring elements used widely, but not only, on production lines of mechanical pieces, for example for regulating the production machines or for quality controls. They are used for an accurate checking of dimensions or for controlling the regularity of a surface or of a production batch.
Trigger probes are usually fastened on the mobile arm of a measuring machine whose position is determined precisely by means of measuring systems, for example capacitive, inductive, magneto-resistive or opto-electronic. The arm of the measuring machine is moved spatially along a determined trajectory until the probe""s stylus comes into contact with the piece or surface to be measured. During contact, a deflective force is applied on the stylus, thus moving it out of its initial resting position. An electric circuit is then either closed or opened and a signal is usually sent on the one hand to the user, for example in the form of a light signal, and on the other hand to the software of the measuring machine that thus determines, on the basis of the data provided by the measuring system, the coordinates of the contact point in a given reference frame.
In another application, trigger probes are used for controlling the regularity of a surface or of a production batch. A probe is fastened on a stationary element and a surface or a series of pieces runs past the probe so as to brush against its stylus. If the surface presents irregularities or if the pieces are not of regular size, the probe""s stylus will be subjected to a force which will move it out of its resting position, thus modifying the signal sent by the probe""s electric circuit.
The main elements of a trigger probe are usually a fixed component, a stylus and an electric circuit.
The fixed component is stationary relative to the fastening element, the latter being usually incorporated to the probe""s casing and enabling the probe to be fastened, for example on the mobile arm of a measuring machine.
The stylus is the mechanical element designed to come into contact with the surface or piece to be measured. The stylus is held by an elastic element in a resting position relative to the fixed component. This resting position is determined and reproducible. The stylus has one or several degrees of freedom relative to the fixed component. During contact with the surface to be measured, a deflective force is exerted on the stylus and will move it out of its resting position according to one or several of its degrees of freedom relative to the fixed component. When the force is no longer applied on the stylus, the latter returns in its resting position.
The electric circuit serves for detecting the movements of the stylus relative to its resting position. The electric circuit has at least one switch that is in a certain state, closed or open, when the stylus is in its resting position and in the opposite state when the stylus is deflected relative to this resting position. The impedance variations of the electric circuit are detected and processed by a signal processing circuit that then transmits the information to a user or, as the case may be, to a computing software that will process the information together with the data provided by a measuring system.
The precision and speed with which a contact between the stylus and a surface or piece to be measured is detected depends for a large part on the precision and speed with which the impedance variation of the electric circuit is detected and processed by the signal processing circuit. But it also depends on the quality of manufacture of the probe""s elements, on the accuracy of their assembling and on their performance through time.
It will thus be easily understood that the positioning and the fastening of the switches"" contact elements play an essential part in this precision.
The contact elements are generally constituted of conductive spherical elements that are placed in their support on the fixed component through a procedure that is long and delicate and consequently costly.
U.S. Pat. No. 4,270,275 proposes for example a trigger probe in which the switches"" contact elements are constituted of balls of conductive material glued, for example with epoxy resin, in a lodging of the fixed component. The connecting elements completing the electric circuit are flexible conductors connecting the switches in series. The document does not explain in which manner the contact is ensured between the balls and the flexible conductors. In the case where the flexible conductors are welded or glued to the balls before assembly, they risk becoming disunited from the balls or to break during the assembly manipulations, and it is difficult to guarantee that the fastening point between the ball and the flexible conductor is oriented correctly at the end of the operation. Furthermore, it is difficult to weld a copper wire on a ball of hard metal and gluing is not reliable. If the wires are simply held in contact with the balls in the glue, the quality of contact cannot be guaranteed and the flexible wires can break through use or escape out of their lodging and thus lose their contact with the balls. Consequently, whatever the solution contemplated, the operation or series of operations is delicate and considerably increases the probe""s price.
U.S. Pat. No. 6,275,053 tries to remedy these problems by proposing contact elements constituted of balls inserted in lodgings of determined depth in the fixed component and held in these lodgings by the casing of the trigger probe. The switches thus constituted are connected in series by the flexible connecting elements compressed between the ball and the casing. Such an embodiment however has several major disadvantages.
First, the balls"" lodgings are of complex shape since they must be sufficiently closed to hold the ball in its position and sufficiently open so that the ball is contacted on one side by a conductive element closing the switch when the stylus is in resting position and on the other side by the flexible connecting element completing the circuit. The dimensions of the lodgings, in particular their depth, must also be sufficiently accurate so that, after assembly of the casing, the ball cannot move in its lodging and so that it exerts a pressure sufficient for establishing a good contact with the flexible connecting element. For these reasons, the manufacture of the fixed component of a probe as described according to different embodiments in U.S. Pat. No. 6,275,053 is costly and delicate.
The assembly of such a probe is also a delicate operation, since all the elements of the electric circuit must be accurately aligned in order for the latter to be correctly constituted and functional.
Furthermore, the contact between the ball and the flexible connecting element being effected by simple pressure, the ball and/or the connecting element will tend to become oxidized with time at their contact point, thus modifying the electric circuit""s impedance until it eventually prevents the probe from functioning properly.
The present invention proposes a trigger probe whose assembly is greatly facilitated by the fact that certain rigid connecting elements constituting the electric circuit are directly united with the switches"" contact elements and contribute to the positioning of these contact elements. During assembly, the contact elements are held in an assembly tool by means of the connecting elements, thus allowing their positioning and their fastening in a single operation. The probe proposed by the present invention also avoids the oxidation problems at the contact points through the fact that the contact elements are welded to the connecting elements.